According to calculations, the Milky Way should contain the remnants of 10 supernovae, but one hasn’t been seen since 1680, when a star went out in a blaze of glory and left behind the remnant called Cassiopeia A.
So, astronomers have been wondering, what is up with that?
Without evidence of young supernova remnants in the Milky Way, astronomers have questioned whether our galaxy, which appears normal, differed in some unknown way from others. Or, the "missing" Milky Way supernovae—exploded stars—indicated they misunderstood the relationship between supernovae and other galactic processes.
Relief came yesterday, when scientists reported they found supernova remains that appear to have been hiding behind a thick veil of gas and dust for 140 years. They are the youngest observed so far in the Milky Way and the first example of a "missing population" of young supernova remnants.
"It's great to finally track one of them down," said David Green of the University of Cambridge in the UK, who used one of two types of instruments to find the object.
According to Stephen Reynolds of North Carolina State University, who observed the object with a second method, the velocities of its explosion debris and extreme energies of its particles are unprecedented. None of the more-mature objects in the galaxy have similar properties, he said.
Supernovae that result from the deaths of stars much more massive than the Sun enrich the galaxy with chemical elements that are produced in the cores of those stars before they explode. The heavy elements, such as carbon, oxygen, iron, and calcium, which make up planets and their inhabitants, were made available by supernova explosions.
In addition to enriching the material between stars with heavy elements, supernovae stir up that material through the shock energy of the explosion. This is thought to help trigger the process of star formation in interstellar clouds of gas and dust. Many astronomers believe our own Solar System is the result of such a supernova shock.
It turns out, Green and his team came across the remnants, now called G1.9+0.3, more than 20 years ago using the Very Large Array (VLA) radio telescope in Socorro, NM, and estimated the object’s age to be 400-1,000 years old. It is near the center of the Milky Way, some 25,000 light-years from Earth.
Another team, led by Stephen Reynolds of North Carolina State University, spotted the object just last year with the Chandra X-Ray Observatory—a satellite instrument that measures x-rays emitted during supernova explosions.
By comparing notes, the astronomers learned the images taken more than two decades apart documented the expansion of debris from the star's explosion. The images taken in 2007 were about 16 percent larger than the ones taken in 1985.
"This is a huge difference," said Reynolds. "It means the explosion debris is expanding very quickly, which in turn means the object is much younger than we originally thought."
Reynolds also observed the object with the VLA radio telescope to confirm the supernova remnant's rapid expansion, using the same method Green did.
Unlike visible-light telescopes, radio and X-ray telescopes can penetrate the thick clouds of gas and dust in our galaxy. "Looking out of the Milky Way, we can see some supernova explosions with optical telescopes across half of the Universe, but when they're in this murk, we can miss them in our own cosmic back yard," Reynolds said.
The astronomers are reporting their results in papers published in the Astrophysical Journal Letters and Monthly Notices of the Royal Astronomical Society.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The Chandra X-Ray Observatory is operated by NASA.
—By Leslie Fink/NSF
This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report.